1. Field of the Invention
The present invention relates to a surface voltmeter and a surface voltage measurement method for measuring a surface voltage on an object.
2. Description of the Background Art
Conventionally, a noncontact-type surface voltmeter has been used for measuring a surface voltage (surface potential) on a photosensitive drum of a copy machine or a substrate having an IC mounted thereon, and some types of surface voltmeters utilize an alternating current (AC) method. In the AC method, a vibrating electrode is positioned close to an object to be measured, and a surface voltage is obtained on the basis of a current from the vibrating electrode which is generated by change of a capacitance between the vibrating electrode and the object. For example, in Japanese Patent Application Laid-Open No. 9-211046 (Document 1), a distance between a vibrating electrode and an object is estimated by detecting change of a current from the vibrating electrode while changing a voltage (electrical potential) of the vibrating electrode, and a surface voltage on the object is obtained by correcting a voltage of the vibrating electrode in a condition where a current from the vibrating electrode is 0 on the basis of the estimated distance.
Japanese Patent Application Laid-Open No. 8-153762 (Document 2) discloses an apparatus having a similar structure to the surface voltmeter, wherein a distance between two objects is changed while applying a voltage to one object and a current from the other object is measured, whereby a difference between work functions of the two objects is obtained.
Since the AC method has merits of easily measuring the surface voltage in a noncontact manner and measuring charge on a surface of an insulating film, it has been started to be applied for preventing charging caused by manufacturing process of a semiconductor substrate and for evaluating an insulating film.
As described above, the conventional surface voltmeter detects a current, which depends on displacement of the electrode, from the vibrating electrode. However, since various parts relating to vibration, such as a cover, are arranged around the vibrating electrode, not only the capacitance between the object and the vibrating electrode but stray capacitances between the parts around the vibrating electrode and the vibrating electrode, change due to vibration of the vibrating electrode. Further, since a range of errors in a measurement result greatly varies according to change of the distance between the vibrating electrode and the object, a complicated operation is necessary for calibration of a measurement value as disclosed in Document 1.
In addition, an object to be measured in the conventional surface voltmeter is a surface voltage on an object which is charged at a high voltage (for example, several kV), therefore, it is not suitable for measurement of a small voltage which is required for evaluation of a semiconductor.